In the related art, general objective lenses for endoscopes have no focusing function but have large observation depths ranging from about 5 to 100 mm from the object side. Endoscopes equipped with such an objective lens provide images using a solid-state image acquisition device, such as a CCD. Recently, to improve diagnostic accuracy, it has been necessary to improve the quality of endoscopic images, and thus, the pixel count of CCDs is increasing. Using high-pixel-count CCDs results in the need to decrease the Fno. of the objective lens to prevent degradation of image quality due to diffraction and to increase the focal length of the objective lens due to an increase in the size of the CCD with an increase in the number of pixels, and thus, the observation depth is being decreased.
This therefore has increased the need for an optical system in which the observation depth can be increased as much as possible and an objective lens having a focusing function to ensure the same level of observation depth as that of a conventional one. A known objective optical system that aims at increasing the depth reduces the focal length while maintaining the same angle of view by using a first lens having a meniscus shape to prevent the occurrence of distortion (for example, see Patent Literature 1). Known objective lenses having a focusing function are constituted by two groups, that is, negative and positive groups, two positive groups, or three groups, that is, negative, positive, and positive groups, respectively, and achieve focusing by moving the second group (for example, see Patent Literatures 2 to 4).
In addition, other known types of magnifying endoscope objective lenses capable of focusing on a nearer object point are constituted by three groups, that is, positive, negative, and positive groups, in which the negative second group moves to achieve focusing (for example, see Patent Literatures 4 to 6). Another known type of objective lens is constituted by three groups, that is, negative, positive, and negative groups, in which the positive second group moves to achieve focusing (for example, see Patent Literature 7).